The invention concerns a film element having a replication layer in which an optically active surface structure is shaped.
Such film elements are frequently used to cause difficulty with or as far as possible prevent copying, forgery or manipulation of security documents and the misuse of such documents. Such film elements are frequently used for safeguarding for example bank notes, credit cards, debit cards, ID documents, tickets, software certificates and the like. In addition such film elements can also be used in the area of trademark and brand protection, for example by those film elements being applied to or integrated into articles, labels or packaging of products in order in that way for example to authenticate the origin of the product.
To make it difficult to copy, forge or manipulate security documents it is known to superimpose a diffractive surface relief which affords optically variable information by diffraction with a thin film layer system generating an optically variable color impression by means of interference. Thus for example WO 01/03945 A1 describes a security product having a transparent substrate, on one side of which is arranged a thin film layer system. The thin film layer system comprises an absorption layer applied to the transparent substrate and a spacer layer comprising a dielectric material. With a suitable choice of the thickness of the spacer layer a viewing angle-dependent color shift effect is generated by interference. A diffractive surface relief is shaped into the transparent substrate on the side opposite to the thin film layer system or also on the side thereof, the diffractive surface relief for example generating the illusion of a three-dimensional image for the viewer. The optical color effect produced by the thin film layer system by means of interference and the optical effects produced by diffraction at the surface relief are superimposed, thereby giving an optical effect which is difficult to imitate and copy. A similar optical security element is also described in WO 02/00445 A1.
In addition U.S. Pat. No. 4,874,213 describes a method of achieving the optical effect which can be attained by a volume hologram, by means of a relief profile which is embossed in a surface. For that purpose the volume hologram is copied into a photosensitive layer and then the Bragg planes of the volume hologram are opened by an opened hologram in a preferably regular pattern so that the volume hologram becomes visible by reflection at the opened surfaces and interference. To avoid as far as possible forgery of the representation of the volume hologram by the opened hologram, the latter is so selected that its diffraction efficiency has a maximum in the zero-order region.